1. Field
The present disclosure relates generally to antennas and in particular to a method and apparatus for transmitting and/or receiving radio frequency energy at different frequencies.
2. Background
Any passive antenna acts identically when used as either a transmitter or a receiver. For the purpose of clarity, most of the following descriptions are related to operation as a receiver.
As a receiver, an antenna converts electromagnetic waves into an electrical current, and as a transmitter, converts an electrical current into electromagnetic waves. An antenna is a structure that may generate an electromagnetic field in response to an alternating voltage and an associated electric current being applied to an antenna when it is functioning as a transmitter. This structure also may generate an alternating current and voltage between the terminals of the antenna when exposed to an electromagnetic field when functioning as a receiver.
Antennas are used in many different systems and applications, such as, for example, communications, wireless local networks, radar, and other systems and applications. For example, antennas may be used on aircraft to provide communications and other functions for the aircraft. In many cases, physical space on an aircraft is limited. Therefore, it is desirable to have an antenna that is as small as possible.
The transmit antenna or the receive antenna may be out of the control of the antenna designer. This situation leads to constraints on the particular antenna being designed. The required gain of an antenna is set by signal-to-noise ratios of the entire radio frequency system, which includes both transmit antennas and receive antennas.
As an example, the designer of an FM radio has no control over the transmitters located at the radio stations. The designer must design a receive antenna for the FM radio that can receive signals over the entire FM radio band with adequate strength. The amount of antenna gain is related to the efficiency and radiation pattern of an antenna. Antenna size is defined here in terms of wavelengths. A small antenna is defined as one that is a fraction of wavelength in size. For small antennas, the radiation pattern is fixed and the gain is related directly to the efficiency.
On an aircraft, the antenna is designed to meet physical space (which is directly related to size) and performance requirements. One way to make an antenna small is to sacrifice bandwidth. With a broadband antenna, a single antenna may be used in place of multiple antennas that operate at different frequencies. Bandwidth restrictions are set by required data rates needed for transmission by the radio frequency system.
An antenna that is small, however, may have significant dissipative (conductive) loss, which reduces the gain. This dissipative loss allows the smaller antenna to operate in a broadband manner, but reduces the efficiency. In fact, losses in gain may be incorporated on purpose to increase the bandwidth of an antenna at the expense of efficiency. Thus, for any antenna type, small antennas are either narrowband or broadband with reduced efficiency.
Therefore, it would be advantageous to have a method and apparatus to overcome the problems described above.